The investigation of the major determinants of monamine synthesis and turnover in vivo is of intense scientific interest because monamine levels in the central nervous system (CNS) play critical roles in neuropsychiatric, neuroendocrine and cardiovascular diseases. Tyrosine and tryptophan hydroxylase are known to the the rate-limiting steps in the syntheses of catecholamines and serotonin, respectively. Current evidence suggests that the in vivo rate of synthesis of these compounds may be mediated by the concentration of reduced biopterin (BH4). Recent reports in the literature have indicated a high correlation between BH4 levels and tyrosine hydroxylase activity in selected brain areas. Our preliminary results indicate significant amounts of BH4 in brain areas known to contain large amounts of tyrosine and tyrptophan hydroxylase. However, it is surprising that we have measured significantly higher amounts of BH4 than would be expected to be found in certain neuroendocrine tissues (e.g., pineal, hypothalamus, pituitary) based on the amounts of hydroxylase enzyme activity in these same tissues.